Air pollution exists not only outdoor, but also indoor. It has great effects on people, and there are many measures taken to cor

Jazz
   Jazz began in the early 20th century as a music of black Americans. It was intended for singing, dancing, and also for entertainment and atmosphere at parties or social gatherings.
   The origin of the word "jazz" is not known, but the term came into common use after the first phonograph records of Jazz were made in 1917. Jazz represents a blending of musical elements from Africa and from Europe. Jazz uses some European ideas of harmony and melody, but the rhythms are more African in origin. It is usually said to be "syncopated", that is, it is irregular in rhythm. Besides this, there are many other rhythms in a piece of Jazz music around one basic rhythm. That is to say, it is polyrhythmic. Another important feature of jazz is improvisation. To improvise means to make something up on the impulse of the moment. This is the way jazz is usually played. Jazz musicians have learned to improvise so well that they can make up excellent melodies as they play. A good jazz soloist seldom plays anything the same way twice. Thus, listening to jazz is not like hearing a piece of music that was written long ago. It is like being there when the music is first being made.
   A major step in the development of jazz was taken by the players in New Orleans. New Orleans jazz, sometimes called Dixieland, was a mixture of different elements. It had the deep emotion of American blues and Negro spiritual, as well as elements of European folk music. The New Orleans style involved improvising that was sometimes complex. Often seven or eight players would improvise at the same time. But the most notable new feature of New Orleans jazz was its more complicated rhythm.
   In the 1940’s a new approach to jazz began to develop. It had been implied in the playing of Lester Young with the Count Basle band. Another important influence on early modern jazz was guitarist Charlie Christian, who. was strongly influenced by Lester Young. At first the new jazz was called bebop, and later, modern jazz. Many of the musical ideas of modern jazz were worked out at gatherings in which the players performed for their own amusement and instruction. Jazz musicians called these gatherings jam sessions. Modern jazz was more complex in harmony and melody than earlier styles of jazz. But a most outstanding feature was its new approach to rhythm. Not only did the players use new rhythms in making their melodies, but the drummers played in a more complex way.
   By the late 1950’s many concert pieces had been written that included parts for jazz improvisation. These pieces usually took the form of a classical concerto with the jazzmen as soloists. Such works are referred to as parts of a "third stream" of music, the first two "!streams" of course being classical music and jazz.
   Since the modern jazz of the 1940’s and 1950’s, jazz has taken another step in its development. This new development is something called "the new thing," or "free jazz." As in earlier major developments in jazz, "free jazz" is characterized by new freshness of rhythm. A whole group of players may change the tempo, or speed, of a piece several times during a performance without planning to do so beforehand.  But this does not mean that the music is disorganized.
   Now we can see that important changes in the development of jazz always include new. ideas of rhythm. The rhythm of New Orleans jazz was new, and so were the rhythm of modern jazz and free jazz. Louis Armstrong and Charlie Parker also brought new ideas of rhythm to jazz. Each style of jazz also had new ways of making melodies, new uses of harmony, and new ways of expressing emotion. Whenever a great player, such as Armstrong or Parker, appears, other players absorb his ideas. Each man then tries to develop those ideas in his own music, in his own way.
   On its way of development, jazz has produced some of America’s leading singers, instrumentalists, and composers, It is continuing to develop. Today, jazz is considered by many to be America’s art music. It has also influenced almost every other kind of music in America, Europe and even the Orient.